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20

VERTEBRAE

ire separated by a certain amount of fibrous tissue forming
;he iutervertebral disc, and the apposed surfaces of the centra
ire as a rule nearly flat. In this last feature, and in the
important fact that the centra are ossified from three distinct
centres, the anterior and posterior pieces (" epiphyses *') remaining
distinct for a time, even for a long time (as in the Whales),
the centra in the mammals differ from those of reptiles and
birds. The epiphyses are not found throughout the vertebral
column of the lowly-organised Monotremata, and they do not
appear to exist in the Sirenia.

Fia. 5.—Anterior sxirface of
Human, thoracic vertebra
(fourth), x f. az, Anterior
zygapophysis ; c, body or
centrum; Z, lamina, and
jp, pedicle, of the neural
arch ; nc3 neural canal; t,
transverse process. (From
Flower's Osteology of the

FIG. 6.—Side view of first
lumbar vertebra of Dog
{Canis familiaris). x j|.
a, Anapophysis ; a«, an-
terior zygapophysis ; in,
metapopliysis ; j?«, pos-
terior zygapophyai.4 ; s,
epinous process ; t, trans-
verse process. (From
Flower's Osteology.}
From each side of the centrum on the dorsal side arises a
process of bone which meets its fellow in the middle line above/
and is from there often prolonged into a spine. A canal is thus
formed which lodges the spinal cord. This arch of bone is
known as the neural arch, and the dorsal process of the same as
the spinous process. The sides of the neural arch bear oval
facets, by which successive vertebrae articulate with one another:
those situated anteriorly are the anterior zygapophyses, while
those on the posterior aspect of the arch are the posterior
zygapophyses; these articular facets do not exist in the tail-
region of many mammals, e.g. Whales.
median   spinous  process  of the